Dry Hopping Effect on Bitterness and IBU Testing

One of the most fascinating hop studies that came out in 2016, was by authors John Maye, Robert Smith, and Jeremy Leker, which found (against traditional wisdom) that dry hopping can influence the bitterness of a beer. I covered the study in detail in an article called, “Increasing Bitterness by Dry Hopping,” which I would recommend reading prior to this article which serves as an update of sorts. In short, the study found that humulinones, which are more soluble than iso-alpha-acids are formed by the oxidation of alpha acids in hops and are 66% as bitter as iso-alpha-acids and have a “smoother” bitterness. The solubility of humulinones is the important part, unlike alpha-acids which are introduced into beer through boiling (becoming iso-alpha-acids), humulinones can be introduced into beer at normal temperatures via dry hopping, which means they can affect the final beer bitterness levels. Alpha acids also dissolve into beer with dry hopping and its concentration can be even higher than humulinones because hops contain so much more alpha-acids than humulinones. However, the bittering potential of alpha-acids is likely around 10% as bitter as iso-alpha-acids.

The authors also found that the starting IBU levels of a beer (IBU levels before dry hopping) also played a role in how humulinones would affect the bitterness. Surprising to me, in high IBU beers, the leaf material in hops absorbed and reduced the amount of iso-alpha-acids in the beer (essentially pulling them out of solution) reducing beer bitterness. In low IBU beers, the hop leaf material is not as efficient at removing iso-alpha-acids yet the humulinones and alpha acids dissolve into the beer making it more bitter. The amount of humulinones in hops was found to be correlated to a hops HSI or hop storage index, which makes sense because of the more oxidized or poorly stored the hop, the higher the HSI and the greater amount of humulinones formed through the oxidation of alpha-acids. Potentially, if a hop has a higher HSI, it might make up for the reduction in iso-alpha-acids when dry hopping higher IBU beers, but not if the hop is fresh with a lower HSI.1

After reading the study I was curious where this line of low IBU beers start and high IBU beers begins since dry hopping can either increase or decrease beer bitterness based on this threshold. In discussions with John Maye, the author of the study, I found out that this “sweet spot” for attempting to determine the effect of humulinones through dry hopping is estimated to be around 25 IBUs. This means that beers with IBUs under 20 can become more bitter by dry hopping and beers with IBUs above 30 can become less bitter by dry hopping. That’s a pretty low threshold, I would guess then that almost every heavily hopped IPA on the market today is actually seeing a reduction in bitterness, but not measured IBUs (the IBU test results go up with dry hopping), unless the brewery uses a really oxidized or high HSI hop, in which case the high humulinones concentration could make up the difference in iso-alpha-acid losses. On the whole, this reducing in bitterness in dry hopped beers isn’t a bad thing in my opinion because of the “smoother” bitterness that humulinones impart.

International Bitterness Test

The international bitterness unit (IBU) test was adopted nearly 50 years ago as the method for determining beer bitterness. In my view, the IBU test has been extremely beneficial in giving brewers a bittering base for both creating recipes as well as replicating results, despite the fact that actual tested IBUs don’t tend to match the estimated figures. This can in part be explained by current trends in brewing hoppy beers like heavy whirlpool/steep additions and dry hopping, which we now know has bittering effects.

The main issue with the IBU test, as noted in a follow-up study on humulinones by Maye later in 2016, is that although it can detect iso-alpha-acids, most of the alpha-acids and most of the humulinones, the IBU test cannot differentiate between them. This is an issue because we now know that humulinones are 66% as bitter as iso-alpha-acids, and alpha acids are about 10% as bitter as iso-alpha-acids, which means the IBU test isn’t accurately measuring the sensory bitterness of a beer based on the varying intensities of bitterness from the different hop bittering compounds. Also, about 12% of the IBU test comes from other hop compounds, perhaps non-bitter polyphenols.

One possible solution according to Maye is high-performance liquid chromatography (HPLC) testing, because it allows for the separation and accurate measuring of the various hop acids in dry hopped beers. For example, through HPLC testing, Maye was able to show that by dry hopping a considerable amount of iso-alpha-acids were removed from beer and replaced by less bitter humulinones and alpha-acids because the test could differentiate between the different hop acids.

By using HPLC to calculate the bitterness of dry hopped beer, you can calculate the bitterness by adding the bitterness intensities of the different hop acids relative to iso-alpha acids. So, an example laid out in the study is a beer that contained 51 ppm of iso-alpha-acids was dry hopped at a rate of 1 1b/bbl with Cascade hops for three days at 60°F. The beer was then tested with HPLC and determined to contain 32 ppm of iso-alpha-acids (reduction of 19 ppm) and 13 ppm of alpha-acids, and 13 ppm of humulinone. Because humulinones are 66% as bitter as iso-alpha-acids, the dry hopping actually added 8.5 ppm of bitterness to the beer. So to put a calculated sensory bitterness figure to the final beer, you can calculate the 32 ppm iso-alpha-acids + (13 ppm humulinone x 0.66) = 40.5 ppm. (Maye, J. P., & Smith, R. (2016). You could improve this sensory bitterness figure even more if you also add the alpha-acids added to the beer via the dry hop (since they are as soluble as humulinones, or more so). Since they are estimated to be about 10% as bitter as iso-alpha-acids, the updated formula would look like this -calculated bitterness (HPLC) = ppm iso-α-acids + (0.66 × ppm humulinone) + (0.10 x ppm alpha-acids). 2

Hop Aroma and Percieved Bitterness

Another interesting variable to sensory or perceived bitterness levels is the effect the “hoppy” aroma can have on an individual’s perception of bitterness. A 2016 study in the Food Research International studied the impact of hop aroma compounds on the perceived bitterness intensity in beer by having experienced and trained tasters sample beers at three different bitterness levels (13, 25, and 42 BU) and spiked with three different levels of hop aroma extract (0, 245, 490 mg/L). The tasters sampled the beers both with and without wearing a pair of nose clips and noted bitterness levels.

The authors found that the addition of hop aroma extract added to the beers resulted in an increase in bitterness perception across the BU levels in the beers. In other words, when the nose clips were off and the hop aroma was apparent, the tasters scored the beers as being more bitter, despite ranking the same beer at a lower bitterness level with the clips on their noses. This was even more true for the medium and high BU beer which had higher bitterness scores without the nose clips at both the medium and high hop aroma dosage, where only the higher dose of hop aroma caused an increase in bitterness perception in the lower 13 BU beer. How could you adjust a bitterness formula to compensate for sensory bitterness by aroma? You probably can’t, but it’s an interesting variable to consider nonetheless. 3

It does seem that current brewing methods and more advanced hop research, particularly with hop acids, has outpaced the old IBU method for testing beer bitterness. It makes sense to me to start moving to a figure that is tied closer to the sensory bitterness that separates out different hop acids and adjusts for the bittering intensities of each rather than just measuring iso-alpha-acids. Perhaps this could be named the Sensory Bitterness Unites (SBU) test?

Summary Points

Dry hopped beers with intense aroma profiles can enhance the perception of bitterness, especially with moderate and high IBU bases.

Beers with IBUs under 20 can become more bitter by dry hopping.

Beers with IBUs above 30 can become less bitter by dry hopping.

Dry hopping will increase the pH of beer, which also increases the bitterness perception of beer.

Humulinones introduced to beer via dry hopping imparts a “smoother” bitterness than iso-alpha-acids and are 66% less as bitter.

Humulinones are more soluble than alpha acids, however, because hops contain so much more alpha acids than humulinones, even if 10% gets into the beer vs 100% humulinones there generally is more alpha in the beer.

It really depends on what you are looking for in a beer and other factors like grist, starting gravity, beer style, etc. I do think you could make a delicious hoppy beer with only doing knockout/whirlpool additions with dry hopping however.

I was thinking an ipa maybe pale ale with very little bitterness but a lot of aroma. Could you do no hops on the hot side just on the cold side, an addition straight into the fermenter with the yeast and then multiple dry hops. Would this provide enough bitterness and a softer bitterness? A beer with no iso-alpha, just alpha and humulones. Anyway just a thought, thank you so much for the blog, great articles.

It’s definitely something I’m interested in trying (currently have one fermenting now that only saw cold side dry hops). We’ll see how it goes, my fear is it will lack that “saturated” hop flavor, but it will be fun see both how it tastes and how the bitterness comes across!

Anecdotal but I brewed a single hop pale ale with Mandarina Bavaria. I planned the beer out so I could can a gallon for future starters. Instead I decided to ferment the extra gallon separately. I dry hopped the main 5 gallons with 1 oz of Mandarina Bavaria and the 1 gallon with .33oz each of Columbus, Cascade and Citra. The ratio obviously being way different, I did this on purpose to get a better feeling for how much weight in dry hopping I really need to use. I also did it to get a good feeling for how mandarina bavaria might work with these hops. At any rate the 5 gallon batch is very balanced with subtle hop aroma and flavor. The 1 gallon batch is a punch in the nose, mouth and tongue. I perceive a dank, not quite intense bitterness with the 1 gallon batch, and very little in the way of bitterness in the 5 gallon batch. I plan to take samples to my homebrew club for some confirmation but there’s really no doubt in my mind that dry hopping adds perceived bitterness at a minimum.

IBU is still a useful measurement as it can be determined with a relatively cheap uv/vis spectrometer which most breweries should be able to afford. HPLC on the other hand is a far more expensive to set up and complex to run and probably beyond the scope of most brewery labs.

Hey Scott,
I appreciate all the research you’ve done on dry hopping, NEIPA, etc. I recently made a batch of NEIPA that has an unexpected sort of harsh bitterness despite having no boil hops. It had a charge of about 1 lb/bbl whirlpool hops at 180° for about 30-40 min and the rest is dryhop. I’ve done beers this way a while and have not had the bitterness issue. Could it be varietal? Anyway, I know you had a blog about this exact thing. Do you have any suggestions for a way to not get this bitterness? On this particular batch I closed the fermenter right after the last dry hop thru the top port. Usually I don’t close as quick and I get a big blow off. Also, I did my first dry hop very close to the begging girls of fermentation. Normally I wait until it’s half attenuated. Could these two differences contribute bitterness? Any info would be greatly appreciated.
Thanks!
Glenn